The term rocket is the collective name given to those craft which include a drive source in the form of a rocket motor. Rockets are currently used, inter alia, for space-flights and for research and communications purposes. Rocket motors are also used in other contexts, for example in apparatus for aiding the take-off of aircraft and for ejecting the pilot from an aircraft in emergency situations.
In a rocket motor, the energy for propulsion of the rocket is generated in a combustion chamber through the burning of a fuel, for example in the form of liquid hydrogen. This fuel is fed together with an oxidizer (for example in the form of liquid oxygen) by means of valves to the combustion chamber. As the fuel burns, combustion gases are generated in the combustion chamber. These combustion gases flow rearwardly out from the combustion chamber, and then out through an outlet nozzle, whereupon a reaction force is generated such that the rocket is propelled forwards. The outlet nozzle is configured to allow for expansion and acceleration of the combustion gases to a high velocity such that the necessary thrust is attained for the rocket. The fact that a rocket motor can generate very large drive forces and can additionally operate independently of the surrounding medium makes it especially suitable as a means of transport for space-flights.
While a rocket motor is running, the outlet nozzle is subjected to very high stresses, for example in the form of a very high temperature on its inside (in the order of magnitude of 800 K) and a very low temperature on its outside (in the order of magnitude of 50 K). As a result of this high thermal load, stringent requirements are placed upon the choice of material, design and manufacture of the outlet nozzle. One of these requirements is a need for effective cooling of the outlet nozzle.
In order to achieve optimal cooling, the outlet nozzles according to the prior art are configured with a number of cooling ducts which are arranged in parallel within the actual nozzle wall, and which extend between the inlet end and the outlet end of the outlet nozzle. The manufacture of the outlet nozzle, that is to say the configuration of its wall such that the necessary cooling ducts are formed, can be carried out using a host of different methods.
In this context, it is also the case that high efficiency can be obtained in a rocket if the cooling medium is also used as fuel. For this reason, there is often a desire to re-use all of the cooling medium for burning in the combustion chamber.
A previously known method of manufacturing a cooled outlet nozzle is by configuring the nozzle wall from a large number of round or oval pipes made, for example, from nickel-based steel or stainless steel, which pipes are arranged close together and are subsequently joined together along their sides. This joining can in this case be realized by means of soldering, which is however a manufacturing method which is relatively costly. Moreover, the soldering results in an increase in the weight of the outlet nozzle. The soldering additionally represents a complicated and time-consuming operation in which it is difficult to attain the necessary strength and reliability in the completed wall structure.
Another significant drawback with solder-based joining is that it is complicated and expensive to check the solder joints. If, for example, a fault occurs along a solder joint, it is very difficult to repair the joint since this damage is not normally accessible. Furthermore, the soldering structure is relatively weak in the tangential direction, which in certain cases can create the need for a strengthening structure in the form of a jacket. This is especially so in those instances in which the flame pressure during the combustion in the rocket motor is very high, or in which high lateral forces are present.
A manufacturing technique which uses soldering can further place a limit upon the maximum temperature at which the outlet nozzle can be used.
An alternative method of manufacturing a cooled outlet nozzle is by diffusion-welding of round or rectangular pipes which are arranged in parallel. Even though this method has advantages over the soldering method, it is still relatively expensive.
According to a further manufacturing method, rectangular pipes of constant cross section made from nickel-based steel or stainless steel are used, which pipes are arranged parallel with one another and are welded together. These pipes are spirally wound such that they form an angle with the geometrical axis of the nozzle, which angle increases progressively from the inlet end of the nozzle to its outlet end, to thus form a bell-shaped nozzle wall. The abovementioned joining method has the drawback that those types of rectangular pipes which are commercially available for use with this method are normally made with a constant wall thickness. This means that the wall structure of the outlet nozzle cannot be configured for an optimal cooling capacity, since the walls between two mutually adjacent cooling ducts are unnecessarily thick. Moreover, the spiral winding means that the cooling ducts are long and hence give rise to an increased fall in pressure, which for certain running states of the rocket motor is undesirable.
A further method for manufacturing a combustion chamber for rocket motors is described in U.S. Pat. No. 5,233,755. According to this method, a corrugated structure is used to form an inner wall, which is joined together with an outer wall by, for example, soldering, diffusion-welding or laser-welding. Cooling ducts are thereby formed, through which a cooling medium can be conducted.
A drawback with the method according to U.S. Pat. No. 5,233,755 is that, owing to the configuration of the corrugated inner wall, “pockets” are formed at its points of contact against the outer wall. In these portions, a limited flow of the cooling medium is therefore obtained, resulting in locally reduced cooling of the wall structure. This gives rise, in turn, to a risk of overheating of the wall structure. There is also a risk of dirt, for example in the form of small particles, accumulating in these pockets. This dirt may subsequently be released from the cooling ducts, which is also a drawback, especially if the cooling medium is also to be used as fuel in the rocket motor.
A further drawback with the manufacturing method according to U.S. Pat. No. 5,233,755 is that the corrugations in the inner wall lead to a limited part of the cooling medium being permitted to have contact with the inner, warm nozzle wall. This too adversely affects the cooling process. Furthermore, the corrugated structure is subjected to bending forces owing to the pressure of the cooling medium inside the structure. Together with the sharp notch at the respective welding joint, these bending forces lead to very high stresses upon the wall structure. This type of structure therefore has limits in terms of its pressure capacity and working life.
The corrugated shape of the distancing material, compared with straight, radially directed distancing elements, leads moreover to increased weight and increased flow resistance.
One object of the present invention is to make available an improved method for manufacturing a cooled outlet nozzle for a rocket motor.